Ground faults in a generator stator winding are the most frequent internal generator electrical fault, and the most frequent cause of damage to the generator stator and a direct cause of phase-to-phase fault of the generator stator. All other electrical faults are always preceded by the ground faults. A ground fault is caused by physical damage to the stator winding or aging of the insulation of the stator. Therefore, stator ground-fault protection is the primary element of a protection system for a generator. Improper operation of ground-fault protection of the stator winding increases the possibility that a ground fault will transform into a phase-to-phase fault, which ultimately will damage the generator.
In order to effectively protect unit-connected generators against the effects of ground faults in the stator windings the phenomena that accompany these faults must be understood. The effects of ground faults depend on energy emitted in a ground-fault channel and on ground-fault over-voltages. By proper operation of ground fault protection it is possible to create conditions under which erosion of the magnetic circuit or stator frame caused by a ground-fault arc is insignificant or totally eliminated. Thus, the occurrence of phase-to-phase faults in the generator is practically impossible, if the ground-fault protection operates properly. During these conditions optimum protection of generator-transformer units against the effects of ground faults in the stator windings is ensured. The repair time for a single line to ground fault should be fairly short since only a stator winding have to be exchanged. The repair time for a phase-to-phase fault may take several months implying a (partial) restacking of the stator core.
In order to minimize the possibility of improper operation of a ground-fault protection system for a generator the particular types of protections forming the system should use different excitation parameters. The neutral point of a generator is not usually directly grounded, and the stator ground-fault currents are then relatively low, especially in generators of generator-transformer units. But even such small currents can cause significant damages to the generator stator iron. They can also transform into phase-to-phase faults.
The method used for protection of a generator against ground faults in the stator winding depends on the method of connecting the generator to the power system. There are several possibilities ranging from directly grounded neutral, different levels of resistive and inductive connection of the neutral, to utilizing isolated neutral. Resistively grounded generators are either connected with a resistor directly or transformed via a neutral grounding transformer utilizing a low voltage resistor on the secondary side of the transformer. A generator directly connected to bus bars and the power system without step-up transformer is usually of low power and the ground-fault protection is then based on supervising the amplitude or direction of the neutral current. A generator connected to the power system via a step-up transformer is theoretically quite simple, because the primary circuit of the step-up transformer is always delta connected and it forms a natural barrier for the ground faults on the higher voltage side. The ground faults on the stator winding can then be detected by monitoring the voltage between the generator neutral point and ground.
There are primarily two methods which in connection with a zero-sequence based protection system may ensure 100% protection for the generator stator winding. One of the methods uses third harmonic in voltages in the generator neutral point and at its terminal. The other method injects a low frequency signal to the generator circuit.
There are three main limitations which influence the length of the stator winding, which is covered by this protection during different operating conditions. They are connected with the following values or parameters of the generator and protection system: Maximum ground-fault current flowing in the generator neutral point; minimum injection voltage which can be measured with good accuracy; and maximum zero-sequence component in the generator neutral point during ground faults in the stator winding. The first and third limitations are independent of the protection system and depends only on parameters of the generator, the grounding system and unit transformer. However, the second limitation can be used to improve ground-fault protection dependent on the protection system.
Further, existing injection methods utilizes an injection frequency below 50 Hz, as higher frequencies increases capacitive current making it difficult to detect changes in the resistance used to detect ground faults.